An annular combustion chamber of a turbomachine comprises an annular end wall of the chamber connected to two shrouds that form two coaxial and substantially cylindrical bodies of revolution extending downstream, together with a fairing or cap that is fastened on the chamber end wall and that extends upstream.
The fairing guides the flow of air delivered by the compressor of the turbomachine and shares it between a central stream that feeds the combustion chamber and two outer streams that flow past the combustion chamber.
The air from the compressor is taken into the combustion chamber and mixed with fuel, and combustion of the mixture is initiated by at least one ignition plug mounted on an outer casing and passing through an orifice in the outer shroud.
In order to seal the combustion chamber at this orifice, it is known to mount guide means in the orifice, which means comprise a tubular guide having the plug passing axially therethrough, the guide being mounted with axial and transverse clearance on a chimney that is fastened to the outer shroud of the combustion chamber and that opens out into the combustion chamber via the above-mentioned orifice.
The clearance allows axial and radial movements to take place between the chamber and the outer casing as a result of differential expansion thereof during different stages of flight, without the plug coming into abutment against or pressing against the edges of the orifice in the outer shroud of the chamber.
Functional clearance also exists between the guide and the plug so that the guide can be moved in translation relative to the plug.
The guide extends in a peripheral stream defined by the outer casing and by the combustion chamber, such that the air flowing in said stream presses the guide against the plug. Contact between the cylindrical inside surface of the guide and the cylindrical outside surface of the plug takes place on the side that is upstream relative to the air flow.
The dimensions of the contact zone are small so the contact pressure or Hertz pressure is high.
In addition, vibration and the above-mentioned air flow tend to cause the guide to turn about the plug.
This gives rise to premature wear of the plug in the zone of contact between the guide and the plug.